12/11/2023 0 Comments Can entropy be negative![]() ![]() The entropy change ΔS of the reaction is negative. Reaction of lime with water takes place without any external help when lime is mixed with water. When lime is reacted with water, hydrated lime is produced. Now the question is, can we predict from the change in the entropy of a reaction between the products and reactants the spontaneity of a reaction? Let us take some examples of typical reactions. We have seen above that from the change in the enthalpy of a reaction, we cannot predict, if the reaction would be a spontaneous reaction. An increase in the degree of disorder of a system shows in a positive value of ΔS. In a gas, the molecules move completely independently of one another in a disordered manner, and the entropy of a gas is therefore always high.Įntropy is given the symbol S, standard entropy is expressed as S0 and change in entropy as ΔS. ![]() When it melts, the ions are free to move, the solid moves to a state of disorder, the entropy increases. Let us take some examples, a crystalline solid, with regular arrangement of ions, has low entropy. The degree of disorder or randomness in a system is measured by a physical quantity called entropy. The common factor in all these four cases of spontaneous reactions is a transition of the system from an ordered arrangement of particles to less ordered arrangement. In the fourth case, when water evaporates, the association of water molecules is replaced by individual water vapor molecules moving independently in vapor phase with much higher kinetic energy. In the third case, when ice melts, the regular hydrogen bonded structure of ice is replaced by less associated fluid which is water. In the second case, when potassium chloride dissolves, the regular arrangement of the crystal structure is replaced by random distribution of mobile ions in solution. When there is combustion of methane and the reaction generates heat there is significant increase of kinetic energy of the product molecules of carbon dioxide and steam, which makes the individual gas molecules moving rapidly in all directions. If we look at all four reactions above, there is a common factor. Question: What makes a reaction spontaneous? Therefore, any enthalpy change in a particular direction, positive or negative, cannot be the only factor to decide if a chemical reaction would take place? There must be some additional factors. We saw in the above reactions that both exothermic and endothermic reactions could be spontaneous. Essentially, it depends on the difference in enthalpy between the products and reactants of a reaction. H2O(s) - > H2O(l), ΔcHo = 6.0 kj / moleĪbove examples show, both exothermic and endothermic reactions could be spontaneous.Įnthalpy (H) is a measure of how much energy is released or absorbed during a chemical reaction. ![]() KCl(s) + aq - > KCl(aq), ΔcHo = 19 kj / mole Similarly there are spontaneous reactions which are endothermic, like It is an exothermic reaction and it is a spontaneous reaction. When ΔG is negative, the reaction would go in the direction of reactants forming products.Ī reaction is called spontaneous when it takes place of its own accord without any external help.ĬH4(g) + 2 O2(g) - > CO2(g) + 2 H2O(g), ΔcHo = −891 kj / mole A spontaneous reaction may involve an increase or decrease in enthalpy, it may involve an increase or decrease in entropy, but it will always involve a decrease in free energy that is a negative ΔG. In this work transfers of the system are found to be frictionless, there is no transfer of matter or of heat and the process is reversible.All spontaneous physical and chemical changes take place in the direction of a decrease in free energy. Note: We must remember that the reversible adiabatic process is also called an isentropic process. It is also found that in the adiabatic process the work done is due to the change in its internal energy.There is no heat transfer take place and the temperature also cannot be valid. System must be perfectly insulated from the encompassing. It is found that the process must be carried out fast so that there is sufficient time available for heat transfer. There are several conditions for the natural process to require place like: In this reaction, the value of entropy change has less than zero. \įor this above given reaction the value of $\Delta ng$ is less than zero. We can write chemical equation for evaporation of water as, The entropy change depends on the heat exchange of the adiabatic system.Īs we know that the entropy change is represented by \ Adiabatic processes can be reversible or irreversible. It is found that this process transfers energy to the surroundings only as work. Hint: We need to know that the adiabatic process occurs without the transfer of mass or heat between a thermodynamics system and its surroundings. ![]()
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